Typically, an imaging spectrometer is composed of an objective or imaging optical module, which forms a scene image at a slit, and a spectral optical module, which receives and collimates the line field of view from the objective, disperses or separates the radiation as a function of wavelength, and images it onto a two dimensional detector array. A double pass technique is sometimes used in the spectral optics. For example, U.S. Pat. No. 5,260,767 discloses the use of the non-relayed reflective triplet optical form as a double-pass, single-channel dispersive spectrometer. For another example, U.S. Pat. No. 7,382,498 discloses the use of the non-relayed reflective triplet optical form as a double-pass, dual-channel dispersive spectrometer. As the reflective triplet optical form is non-reimaging (non-relayed), when used for infrared applications, the entire spectrometer optical train must be cooled, generally inside a cryo-vac housing. This requirement results in a cool-down time measuring several hours, which may severely limit the use of the instrument in certain applications or environments.